The “Blue” Moon isn’t really blue. The term comes from an old English usage that originally meant “betrayer” Moon, which was a Full Moon that appeared when it shouldn’t have. This all had to do with the church’s calculation of Lent and other religious holidays, so an extra Full Moon during the year caused some problems. The actual definition of a Blue Moon is the third Full Moon in any season that has four Full Moons. (Most seasons have only three.) These days, however, most people call the second Full Moon in a month a Blue Moon. (8-inch Celestron Schmidt-Cassegrain telescope at f/10, Canon 6D, ISO 400, 1/640-second exposure, taken July 31, 2015, from Dayton, Ohio)